We are studying the molecular characteristics of the oncoplacental protein SP1 and its expression in some in vivo and in vitro conditions. SP1 is found in the serum of pregnant women, of most patients with gestational trophoblastic tumors, and of some patients with germ cell tumors. SP1 can neither be found in the circulation of nonpregnant healthy individuals nor of patients with cancer of the digestive tract, breast cancer, melanoma and sarcoma. SP1 is consequently not a general tumor marker, but assays for SP1 may be useful in monitoring patients with tumors of the reproductive system. In vitro, all skin fibroblasts from normal adults have been found to produce SP1. In contrast, most human tumor cell lines tested, including some of trophoblastic origin, did not produce detectable amounts of SP1. Only glioma was found to produce SP1 in vitro, and the production of SP1 was retained when the tumor was growing in nude mice. The apparent discrepancy between the in vivo and in vitro expression of SP1 should provide interesting insights into the regulation of expression of oncodevelopmental proteins once its molecular basis is understood.